During the second half of 2016, the code basically stayed untouched. In
early 2017, several patch series of (at least) three authors have been
published on the netdev mailing list for review and merge.

This poses the very valid question on how do we test those (sometimes
quite intrusive) changes. Setting up a complete cellular network with
either GPRS/EGPRS or even UMTS/HSPA is possible using OsmoSGSN and
related Osmocom components. But it's of course a luxury that not many
Linux kernel networking hackers have, as it involves the availability of
a supported GSM BTS or UMTS hNodeB. And even if that is available,
there's still the issue of having a spectrum license, or a wired setup
with coaxial cable.

So as part of the recent discussions on netdev, I tested and described a
minimal test setup using libgtpnl, OpenGGSN and sgsnemu.

This setup will start a mobile station + SGSN emulator inside a Linux
network namespace, which talks GTP-C to OpenGGSN on the host, as well as
GTP-U to the Linux kernel GTP-U implementation.

This is of course just for manual testing, and for functional (not
performance) testing only. It would be great if somebody would pick up
on my recent mail containing some suggestions about an automatic
regression testing setup for the kernel GTP-U code. I have way
too many spare-time projects in desperate need of some attention to work
on this myself. And unfortunately, none of the telecom operators (who
are the ones benefiting most from a Free Software accelerated GTP-U
implementation) seems to be interested in at least co-funding or
otherwise contributing to this effort :/